Fuel injection valves of this type with indirect electromagnetic control of the nozzle needle of the fuel injection valve are revealed, for example, in the following publications:
U.S. Pat. No. 4,566,416, EP-A-0 228 578 and EP-A-0 262 539.
In the fuel injection valves revealed in the publications quoted above and in the fuel injection valve of the present invention, the nozzle needle is indirectly actuated electromagnetically by means of a hydraulic amplifier. The fuel pressure in a control space acts on a needle piston and holds the nozzle needle closed. The control space is connected to the high-pressure supply conduit of the fuel injection valve via a first throttle hole and the control space can be relieved via a second throttle hole. The outlet from this second throttle hole can be opened and closed by an electromagnetically actuated pilot valve. If the pilot valve is actuated, the second throttle hole is opened. Because of the first throttle hole, the pressure in the control space drops. In consequence, a force occurs on the nozzle needle in the opening direction of the nozzle needle and injection begins. If the pilot valve is closed again, the pressure in the control space builds up again, the nozzle needle is closed and the injection is, in consequence, ended. Fuel injection valves of this type are therefore suitable for generating intermittent injections such as are necessary, for example, in the case of diesel engines.
The maximum injection pressures of these injection valves can be more than 1000 bar. The minimum injection pressures vary between 100 and 300 bar. The engine is operated under load and up to full load by using the upper injection pressure range whereas the engine is operated at idle and at very low load in the lower pressure range.
In order to achieve good power and exhaust gas figures from the engine, the injection period must be short under load and at high rotational speed of the engine. This period is generally about one-thousandth of a second. The quantity injected at these operating points is substantially larger than that when the engine is idling. The fuel quantity required per working stroke at idle is, on the contrary, extremely small because it is only necessary to overcome the friction of the engine and there is no power output at the drive end of the crankshaft. In addition, the rotational speed of the engine is low. In order to avoid rough and noisy engine running at idle and at low load, it is desirable for the injection period to be relatively long, despite the small injection quantity. It should again, typically, be between one-thousandth and two-thousandths of a second.
Because the injection quantity is small, it is difficult to generate a long injection period with the fuel injection valves of known type and it is, in consequence, difficult to achieve quiet, smooth engine idling.